The regulation of herpes simplex virus immediate early gene (IE) expression is governed by viral and cellular proteins that assemble a multiprotein transcription enhancer complex. The analysis of the protein components and the biochemical interactions provides a model for RNAPII directed transcription and identifies critical elements of the HSV IE gene regulatory mechanism. One such component, the cellular C1 factor, is a large and complex family of polypeptides which functions to direct the assembly of the enhancer complex as well as to mediate the transcriptional activation potential of some of the associated polypeptides. In this context, a direct interaction of the C1 factor with GABP has been demonstrated, defining the C1 factor as the critical coordinator of the enhancer complex assembly. These studies have indicated that the C1 factor functions as a novel coactivator of GABP-mediated transcription. The interaction and coordinated assembly of the enhancer proteins by the C1 factor may be critical for the regulation of the HSV lytic-latent cycle. In a similar manner, the C1 factor may play important roles in the regulation of cellular transcription as evidenced by the identification and isolation of numerous cellular transcription factors that interact with the protein.